This invention relates generally to a method of replacing existing pipe-type power cables with extruded-dielectric cables.
Pipe-type power cables have been the backbone of underground transmission systems in North America for more than 80 years. It is estimated that the United States and Canada have between 3000 and 4000 circuit miles of underground buried steel pipe which contain three-phase paper insulated power cables. Some estimates indicate that more than one-half of this amount of buried pipe-type cables are more than 50 years old.
The cables inside the pipe are provided individually, one for each phase. Each metallic conductor is insulated with a paper-based insulation. The space inside of the pipe with the cables is filled with a dielectric fluid. This design has proven to be robust and reliable; however, there are some drawbacks: (1) the fluid-filled configuration is not considered to be environmentally friendly in the event of a leak; and (2) there are very few companies which manufacture the cables that go into the buried pipe; thus replacing a failed pipe-type cable or reconductoring the pipe with pipe-type cable may not be feasible in the future.
One solution is to replace the pipe-type cables with extruded-dielectric cables, still using the existing pipe. Extruded-dielectric cables do not require dielectric fluid and have both operational and maintenance advantages. Extruded-dielectric cables use a polymer-based insulation. However, based on the current industry requirements, the insulation wall thickness and the outside diameter of each cable using the polymer insulation and the same metallic conductor diameter are greater than those of the paper-based insulated cable, respectively. Therefore, to replace the paper-insulated cables with the polymer-insulated cables, the diameter of the metallic conductor of the polymer-insulated cable would be reduced, if the same existing buried pipe is used. The reduction of the metallic conductor diameter would result in an electric current capacity reduction. In addition, the steel pipe that surrounds the polymer-insulated cable has magnetic losses, and the pipe causes an increase in the electrical losses of the polymer-insulated cables themselves. The heat generated by these losses further reduces the electric current capacity.
On occasion, utilities have replaced the paper insulated cables with extruded-dielectric cables, accepting the lower circuit ratings; however, such reduction is not acceptable by most end-users who wish to have the same or greater current carrying capacity. As a result, the utilization of extruded-dielectric cable with existing buried steel pipe has been impeded.
Accordingly, there is a need for a method of replacing existing pipe-type cable systems with pipe and/or conduit of larger diameters to accommodate the installation of extruded-dielectric cables that can meet power transfer demands equal to or greater than the existing pipe-type cable circuits.